Cuando voy en moto por la ciudad, nunca olvido el casco.

In Spanish, the simple present is very commonly used for habitual actions:

• Cuando voy en moto por la ciudad... = When I ride/go by motorbike through the city...

Using estoy yendo would sound much more like something happening right now, and it is much less natural here. Since the sentence describes a general habit, voy is the normal choice.

Ir en + vehicle means to go by / to travel in or on a vehicle.

So:

• voy en moto = I go by motorbike / I ride a motorbike

Other examples:

• voy en coche = I go by car
• voy en tren = I go by train
• voy en autobús = I go by bus

Even though English often says I ride a motorbike, Spanish usually uses ir en moto in this kind of context.

Because Spanish normally uses en with means of transport:

• en coche
• en tren
• en avión
• en moto

Using a moto would not be correct here. Think of en as the standard preposition used with transport in Spanish.

Here, por la ciudad means something like:

• around the city
• through the city
• in the city area, moving through it

Por often suggests movement through or around a place.

Compare:

• en la ciudad = in the city
• por la ciudad = through/around the city

Since the sentence is about travelling on a motorbike, por la ciudad sounds more natural because it emphasizes movement through the city streets.

Cuando means when. Here it introduces a situation that happens regularly:

• Cuando voy en moto por la ciudad, nunca olvido el casco.
• When I go by motorbike through the city, I never forget my helmet.

This is a very common structure in Spanish for habits and repeated situations:

• Cuando trabajo, escucho música.
• When I work, I listen to music.

Because olvidar can be used transitively when you forget a thing directly:

• Olvido el casco. = I forget the helmet.

You may also hear olvidarse de:

• Me olvido del casco.

Both are possible, but they are structured differently:

• olvidar + noun
• olvidarse de + noun

In your sentence, olvido el casco is perfectly natural and straightforward.

Spanish often uses the definite article (el, la, los, las) where English would use a possessive like my.

So:

• olvido el casco literally = I forget the helmet
• natural English = I forget my helmet

Because it is obvious whose helmet it is, Spanish does not need mi. This happens a lot with clothing and personal items:

• Me lavo las manos. = I wash my hands.
• Me pongo la chaqueta. = I put on my jacket.

In this sentence, yes, casco means helmet.

But casco can have other meanings in different contexts, for example:

• a helmet
• a shell/husk
• the old town or central part of a city in expressions like casco antiguo

Here, because of moto, the meaning is clearly helmet.

Nunca means never, and it normally goes before the verb:

• Nunca olvido el casco.
• I never forget the helmet.

That is the most neutral word order.

You can also sometimes place it elsewhere for emphasis, but nunca olvido is the standard pattern.

It is talking about a general habit.

The Spanish present tense often covers:

• what you do now
• what you do regularly
• general truths

So here:

• Cuando voy en moto por la ciudad, nunca olvido el casco.

means something like:

• Whenever I ride my motorbike in the city, I never forget my helmet.

It is not just about one specific trip happening at this moment.

Yes, but the meaning changes slightly.

• en moto = by motorbike, by motorcycle in general
• en la moto = on the motorbike / on that specific motorbike

So in your sentence, en moto is better because it refers to the means of transport in general, not a particular bike.

The comma separates the introductory cuando clause from the main clause:

• Cuando voy en moto por la ciudad, / nunca olvido el casco.

This is very normal in Spanish writing, just as in English:

• When I go by motorbike through the city, I never forget my helmet.

It helps make the sentence easier to read.

Yes, absolutely. That word order is also natural.

Both are correct:

• Cuando voy en moto por la ciudad, nunca olvido el casco.
• Cuando voy por la ciudad en moto, nunca olvido el casco.

The difference is mainly one of focus and rhythm:

• en moto por la ciudad highlights the transport first
• por la ciudad en moto highlights moving through the city first

Both would be understood easily.